The invention relates to a ballast arrangement for a discharge lamp with resonant ignition, comprising a coil which is situated in a diagonal branch of a bridge-shaped commutation circuit, and which is arranged in series with the electrical lamp connections, and comprising a capacitor arranged in parallel with these connections, which commutation circuit supplies a comparatively low-frequency square-wave supply voltage of comparatively small amplitude to the lamp, and, in an ignition phase preceding the normal operating phase, the commutation circuit is commutated at a comparatively high frequency by a switching voltage at a switching frequency that changes with respect to time.
The invention more particularly relates to a ballast arrangement with resonant ignition of the lamp, wherein an ignition voltage causing ignition of the lamp is brought about across the electric lamp connections by electrically making resonant said coil and said capacitor. Such a ballast arrangement is disclosed, for example, in European patent application EP 0408121. In said application, it is described that the commutation circuit is commutated in an ignition phase at a time-variable switching frequency the frequency of which passes the resonant frequency of said coil and capacitor at some moment in time. It is an object of the invention to sufficiently heat the electrodes of the lamp in a so-termed take-over/warm-up phase between the resonant-ignition phase and the normal operating phase by means of glow discharges so as to attain a reliable transition to the normal operating phase without the supply circuit, which in turn feeds the commutation circuit, being excessively loaded.
According to U.S. Pat. No. 5,932,976, which also relates to resonant ignition and wherein also a circuit frequency of the commutation circuit which sweeps in frequency is employed, at the end of the frequency sweep, the frequency is maintained for some time at a frequency value close to the resonant frequency of the coil and the capacitor. As a result, however, the supply circuit, which in turn feeds the commutation circuit, is heavily loaded and the electrodes of the lamp are not effectively preheated.